A continuous-time delta-sigma ADC (Analog to Digital Converter) always includes a resonator having a single op-amp (operational amplifier). The resonator determines notch points that can suppress noise. However, the notch points are determined by gain values for the op-amp included in the resonator, and it is difficult to achieve high gain via single op-amp. The performance loss due to finite op-amp gain may no longer be neglected for high dynamic range device.
FIG. 1 is a circuit diagram illustrating a related art resonator having a single operational amplifier. As shown in FIG. 1, the input signal V1n has been processed by the integrator 101 before enters the resonator 103. The resonator 103 includes an op-amp OP and a plurality of resistors RC2-RC5 and CC2-CC5. As above-mentioned description, high gain can not be provided by only one op-amp, thus the resonator 103 has above-mentioned disadvantage. Some related arts provide resonators having more than one op-amps, such resonator also occupies a large circuit region, however.